1. Field of the Invention
The present invention relates to a sound synthesis system, having a pitch adjusting function, which comprises a loop containing a delay portion and which adjusts pitches of sounds to be synthesized.
2. Prior Art
U.S. Pat. No. 5,212,334 discloses an example of the sound synthesis system, using a digital signal processor (i.e., DSP), which simulates a tone-generation mechanism of an acoustic musical instrument so as to synthesize musical tones of the acoustic musical instrument. An example of the sound synthesis system conventionally known will be described with reference to FIGS. 4 and 5 which show a musical tone synthesizer providing a pitch adjusting function. For convenience's sake, the musical tone synthesizer of FIGS. 4 and 5 is configured by circuit elements. However, functions of the musical tone synthesizer can be embodied by programs which are executed by the DSP. Or the musical tone synthesizer can be configured by other electronic circuits which execute algorithms of the programs.
Now, the musical tone synthesizer of FIG. 4 is designed to simulate sounds of wind instruments such as a clarinet. An overall system of the musical tone synthesizer is divided into three sections, i.e., an excitation circuit 110, a junction 120 and a resonance circuit 130. The excitation circuit 110 is provided to simulate operations of a mouthpiece of the wind instrument; the resonance circuit 130 is provided to simulate operations of a resonance pipe of the wind instrument; and the junction 120 is provided to simulate a scattering manner of air-pressure waves at a connection part between the mouthpiece and resonance pipe of the wind instrument.
The excitation circuit 110 comprises a subtracter 111, a filter 112, an adder 114, a non-linear circuit 115, multipliers 116, 117 and an inverter `INV`. The subtracter 111 receives a feedback signal which is outputted from the resonance circuit 130 and is transmitted thereto through the junction 120; and the subtracter 111 also receives a blowing-pressure signal `PRES` which corresponds to blowing pressure applied to the mouthpiece of the wind instrument. Based upon the feedback signal and the blowing-pressure signal PRES, the subtracter 111 performs a calculation to produce a signal which corresponds to air pressure imparted to a reed of the wind instrument. Then, the signal produced is supplied to the filter 112 and is also supplied to the multiplier 116 through the inverter INV. The filter 112 is incorporated into the loop in order to avoid an event in which specific frequency of a signal circulating between the excitation circuit 110 and the resonance circuit 130 does not increase remarkably. An output `P.sub.1 ` of the filter 112 is supplied to the adder 114 in which it is added to an embouchure signal `EMBS`. Result of addition of the adder 114, i.e., a signal `P.sub.2 `, is supplied to the non-linear circuit 115. The embouchure signal EMBS is a signal which corresponds to open/close manners of lips or shaping of lips. The signal P2 outputted from the adder 114 corresponds to pressure imparted to the reed of the wind instrument.
Based upon the signal P.sub.2, the non-linear circuit 115 creates a signal `Y` corresponding to admittance against air flow which occurs at a gap between the reed and mouthpiece. The signal outputted from the subtracter 111 is inverted to a signal `-PA` by the inverter INV. The multiplier 116 performs a multiplication on the signals Y and -PA so as to produce a signal `FL` which corresponds to velocity of the air flow passing through the gap between the reed and mouthpiece. This signal FL is supplied to the multiplier 117 in which it is multiplied by a multiplier `G`, wherein the multiplier G is determined responsive to a pipe diameter. Result of multiplication performed by the multiplier 117 is a signal corresponding to a variation of air pressure in the resonance pipe at a certain position which is close to the mouthpiece.
The above signal is supplied to the adder 118 in which it is added to the aforementioned feedback signal. An output signal of the adder 118 is supplied to the adder 119 to which the feedback signal is supplied as well. The junction 120 is configured by the adders 118 and 119 as well as cross-line connection between them.
The output signal of the adder 118 is supplied to the resonance circuit 130 in which it is supplied to the delay circuit 121. The delay circuit 121 is provided to simulate a delay (i.e., a delay time `DL`) between a moment, at which an air-pressure wave is produced by the reed, and a moment at which the air-pressure wave reaches a tone hole. Herein, the tone hole is provided to determine pitch of a sound of the wind instrument. Thus, the delay circuit 121 determines pitch of a musical tone synthesized. A delayed output of the delay circuit 121 is used as the aforementioned feedback signal. The feedback signal is inverted by the inverter IV and is then supplied to the filter 122. The filter 122 performs frequency-band restriction on the feedback signal; thereafter, the feedback signal is outputted from the resonance circuit 130 and is then supplied to the junction 120.
In the junction 120, the feedback signal is added to the output signal of the adder 118 by the adder 119. Then, result of addition of the adder 119 is outputted from the junction 120 and is then supplied to the excitation circuit 110 in which it is transmitted to the subtracter 111. As described above, the excitation signal circulates between the excitation circuit and the resonance circuit 130 through the junction 120. Thus, a waveform signal `WS` is outputted from the delay circuit 121.
According to the above description, pitch of the waveform signal WS is determined by the delay time DL of the delay circuit 121, wherein the waveform signal WS is a signal representing a musical tone synthesized. Actually, however, the pitch of the waveform signal WS is determined by a total amount of delay of the loop consisting of the excitation circuit 110, the junction 120 and the resonance circuit 130. There are provided the filters 112 and 122 in the loop; and in general, a delay time of the filter is changed responsive to its filter coefficient. For this reason, the delay time DL of the delay circuit 121 should be determined under the consideration of the filter coefficients; otherwise, pitch of a musical tone synthesized must be deviated from a desired pitch. However, in order to compute a delay time of the filter based on the filter coefficient, it is necessary to perform complicated computation and/or complicated processing which requires much time. In order to cope with such a problem, the conventional technology provides a pitch adjusting portion, as shown in FIG. 5, which is incorporated into the musical tone synthesizer of FIG. 4.
In FIG. 5, the waveform signal WS is supplied to a band-pass filter 104 which is controlled to extract only a pitch of a musical tone to be produced. In other words, unnecessary waveform components are removed by the band-pass filter 104. Then, the waveform signal WS is supplied to a clipper 103. The clipper 103 shapes the waveform signal WS into a rectangular-wave signal `SC2`, which is then supplied to one input of a phase comparator 102.
An oscillation circuit 101 performs an oscillation to produce a pulse signal `SC1` based on a signal `OF` which corresponds to a keycode. The pulse signal SC1 is supplied to another input of the phase comparator 102 in which it is compared with the rectangular-wave signal SC2. The phase comparator 102 produces a phase-difference signal representative of a phase difference between those signals SC1 and SC2. A low-pass filter 105 smoothes the phase-difference signal to produce an error signal which is then supplied to a conversion circuit 107. The conversion circuit 107 converts the error signal into a signal which corresponds to a delay length.
A delay-length signal outputted from the conversion circuit 107 is supplied to an adder 108 in which it is added to an initial delay-length signal `d1`. Thus, correction to the delay-length signal is performed. The adder 108 outputs a corrected delay-length signal `DL`. This corrected delay-length signal DL is supplied to the delay circuit 121. When the pitch of the waveform signal WS coincides with the keycode, the error signal of the low-pass filter 105 is set at zero. When a synchronization detecting circuit 106, which monitors the error signal, detects an event in which the error signal is set at zero, the synchronization detecting circuit 106 outputs an pitch-adjustment end signal `END` which declares that an adjustment to the pitch is ended.
As described above, phase comparison is performed between a certain reference signal `SC1`, created by the oscillation circuit 101, and the signal `SC2` corresponding to the waveform signal WS to be controlled; and then, the pitch adjusting portion of FIG. 5 controls the loop by adjusting the delay time `DL`0 of the delay circuit 121 in such a way that the phase difference between them is eliminated. Thus, the pitch adjusting portion of FIG. 5 is designed based on a so-called PLL system (wherein `PLL` is an abbreviation for `Phase Locked Loop`). However, the pitch adjusting portion based on the PLL system is disadvantageous in that much time is required to perform a pitch adjustment; in other words, it is impossible to perform the pitch adjustment on musical tone signals in real time during a progression of musical performance. Due to such disadvantage, pitch adjusting operations should be performed with respect to all pitches of an electronic musical instrument before the musical performance is actually played. Thereafter, results of the pitch adjusting operations are used to obtain data representative of the corrected delay-length signals `DL`; and then, a table is created using the data. This table is stored by some storage device.In the musical performance actually played, a desired corrected delay-length signal is read from the table in response to a keycode designated, so that the corrected delay-length signal is supplied to the delay circuit 121.
The pitch adjusting portion based on the PLL system has a complicated configuration. In addition, the pitch adjusting portion performs a pitch adjustment based on the phase difference between the reference signal (i.e., SC1) and a musical tone signal created by the musical tone synthesizer which comprises the loop containing the delay circuit. Thus, the conventional technology suffers from a problem that the pitch adjustment is hard to be performed at high speed and with high accuracy.
In addition, a musical tone waveform is a complicated waveform. Therefore, it is difficult to detect a phase of the musical tone waveform. In other words, it is difficult to detect a phase difference between the reference signal and musical tone waveform with accuracy. For this reason, it is impossible to perform a high-precision pitch adjustment with ease.